1. Field of the Invention
This invention relates to an apparatus for processing of television camera signals used in close circuit television for recording and monitoring system.
2. Description of the Prior Art
In information signal processing systems, such as a television monitoring system, a video cassette recorder or a digital video recorder is connected to a television camera for recording television camera signals at a time lapse or at a real time. A video cassette recorder or a digital video recorder may also be connected through an electronic switch, known as a multiplexer, to a plurality of television cameras via plurality of transmission lines. The electronic switch or the multiplexer provides for a fast rate sequential switching for recording onto the video recorder a picture image of one camera after another and for selectively displaying the playback or monitoring each of individual camera signals, or for combining multiple camera signals or multiple recorded stored signals into a multi screen display, for example four, nine or sixteen cameras onto one display known as a split screen display. Such a system and method and apparatus are disclosed in U.S. application Ser. No. 09/818,243 filed Mar. 27, 2001.
Since the multiple camera signals are recorded one after another in serial sequence, it is not possible to record any one of plurality camera at real time, such as the 30 frames or the 60 fields per second, used in the NTSC standard.
Moreover, non externally synchronized television cameras generate their field or frame sync at random, requiring the detection by the digital video recorder of the vertical sync signal, pertaining to each of the signals of sequencing television camera signals and therefore start the recording or monitoring of each such field or frame at random. This causes random delays in processing of each field or frame processing, thereby causing a longer lapse time in recording or displaying of each individual camera signal. This causes also a slower refresh rate for the split screen displays.
Therefore in such an information processing system, it is preferable to mutually lock the internal synchronizing signals of all of a plurality of connected television cameras and the electronic switching time to an external synchronizing signal.
A known apparatus for synchronizing a plurality of television cameras is an apparatus for transmitting an external synchronizing signal from an external synchronizing generator to the television cameras by injecting the external synchronizing signal into the video signal transmission line and locking a generator of an internal synchronizing signal of the television camera by means of the transmitted external synchronizing signal. Such an apparatus is disclosed in U.S. Pat. No. 4,603,352 the entire content of which is incorporated herein by reference.
Another known apparatus for synchronizing a plurality of signals of the television camera is an apparatus for memorizing each signal of the television camera in a memory circuit and feeding the signals to the video recorder, synchronously via the electronic switch from the memory circuits. Such an apparatus is disclosed in U.S. Pat. No. 5,267,039 the entire content of which is incorporated herein by reference.
Furthermore, the known apparatuses for recording a plurality of camera signals at fast sequencing rate require that each individual camera signal will incorporate a coded identification signal, providing for simple retrieval of the signal through a coded selection, such identification code is disclosed in U.S. Pat. No. 4,943,864 the entire content of which is incorporated herein by reference. In the case of using analogue video cassette recorder, the coded identification is known to be recorded during the vertical blanking period of television signals, but in the case of using a digital video recorder it is known that digital video recorders ignore the vertical blanking period and instead they employ a complex control software for indexing the files of the recorded digital signals of each individual picture.
Whenever a single television camera is connected to a digital video recorder, the digital video recorder synchronizes itself to the synchronizing signal generated by the camera and therefore can process and record a camera signal at real time, but however limits the monitoring of the signal and recording and the play back to one camera at a time only.
In systems employing a plurality of digital video recorders each connected to a single camera it is not possible to combine efficiently the output signals of the digital video recorders into the split screen displays for monitoring or playback because the digital video recorders are not externally synchronized and locked to each other, and therefore generate their field or frame at random.
Furthermore, the output signals of digital video recorders are display signals consisting of red, green and blue components that are commonly fed to computer screens known as VGA. Such VGA output signals fed from multiple digital video recorders are complex and costly to combine into split screen displays and they are therefore never used for split screen combination.
On the other hand, some digital video recorders convert the recorded output signals into composite video signal. However, as stated above, in a system comprising a plurality of digital video recorders, their output signals are not externally synchronized, nor are the signals coded for identification, and therefore in order to display those signals on a split screen, the signals must be coded, their signal sync must be detected and merging of those signals takes place at random timing, which causes a slow refresh rate and lower picture quality.